Things to look out for...

Surf via Wi-Fi or a Wi-Fi Hotspot rather than mobile Internet.

Use apps offline.

When connected to Wi-Fi, make your streaming services (e.g. YouTube, Spotify and Apple Music) and cloud apps (e.g. OneDrive, Dropbox and iCloud) available offline. That way, you don't use any data for these apps while on the move.

Don't play videos or music in the highest quality.

If you want to play videos or music via the mobile network, change the quality in the app settings. Don't allow apps such as Facebook, Instagram or WhatsApp to automatically play or download videos.

Don't allow updates of your e-mail, calendar, contacts, Facebook messages, or your OneDrive, Dropbox or iCloud files to be synced in the background.

For iCloud:

Go to Settings in the main menu and select iCloud.

Move the slider next to Use cellular data to the left to disable this option.

If you don't see this option, you can disable mobile data completely for iCloud (or other apps). For this, return to Settings and choose Cellular. Move the slider next to iCloud to the left to disable this option.